[9.5](backport #7568) fix: size ristretto cache from GOMEMLIMIT instead of host RAM

[mergify]

What is the problem this PR solves?

memEnvLimits() in internal/pkg/config/env_defaults.go calls memory.TotalMemory() to select the ristretto cache tier. memory.TotalMemory() returns the host node's total physical RAM, not the container's cgroup memory limit. On a Kubernetes node with ≥16 GB of RAM, this selects a MaxCost of 256–512 MB — 2–4× the pod's GOMEMLIMIT (128 MB at the 256 M default pod limit). The cache is allowed to grow past the GOMEMLIMIT, triggering OOMKills even when live application heap is within budget.

This was the primary driver of the OOMKills observed for a high-volume serverless project in elastic/ingest-dev#8991.

How does this PR solve the problem?

Introduces containerMemoryMB(), which reads the current GOMEMLIMIT via debug.SetMemoryLimit(-1) and converts it to MiB. When GOMEMLIMIT is set (i.e. ≠ math.MaxInt64), that value drives cache tier selection — so a pod with a 256 M limit and GOMEMLIMIT = 128 M will select a cache sized for 128 MB of available memory, not 16+ GB of node RAM.

When GOMEMLIMIT is unset (math.MaxInt64), the function falls back to memory.TotalMemory(), preserving the existing behaviour for non-containerised deployments.

memMB remains a var pointing to containerMemoryMB, so existing tests that stub memMB are unaffected.

How to test this PR locally

Run the config package tests:

go test ./internal/pkg/config/... -run TestContainerMemoryMB -v

TestContainerMemoryMB covers both the GOMEMLIMIT path (sets a 256 MiB limit and asserts the function returns 256) and the fallback path (clears the limit and asserts the function returns host RAM in MiB).

To verify the end-to-end effect: start fleet-server with GOMEMLIMIT=128MiB (or via server.runtime.memory_limit) and observe that the logged recommended_mb value is ≤ 128, not 256–512.

Design Checklist

• I have ensured my design is stateless and will work when multiple fleet-server instances are behind a load balancer.
• I have or intend to scale test my changes, ensuring it will work reliably with 100K+ agents connected.
• I have included fail safe mechanisms to limit the load on fleet-server: rate limiting, circuit breakers, caching, load shedding, etc.

Checklist

• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in ./changelog/fragments using the changelog tool

Related issues

• Closes elastic/ingest-dev#8991

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  This is an automatic backport of pull request fix: size ristretto cache from GOMEMLIMIT instead of host RAM #7568 done by Mergify.